This invention relates to a process for producing high-purity ruthenium useful as a material for the lower electrode of ferroelectric capacitors for MOS-ULSI, and relates also to high-purity ruthenium sputtering targets.
In recent years, research has been made in the manufacture of semiconductor memories about the use of thin films of such ferroelectrics as Ba--Ti, Sr--Ti, and Ba--Sr--Ti complex oxides as capacitors on wafers of silicon or the like as semiconductor elements. Study is also under way on the formation of a ruthenium oxide film by the sputtering of a ruthenium target material in an oxygen atmosphere as a lower electrode material for those thin film ferroelectric capacitors.
In order to ensure that such a semiconductor member formed by sputtering should perform reliably, it is important to minimize the proportion of the metal and other impurities in the material that are deleterious to the semiconductor device.
The impurities that must be removed to minimum contents include:
(1) alkali metal elements such as Na and K; PA1 (2) radioactive elements such as U and Th; and PA1 (3) heavy metal elements such as Fe and Ni.
Alkali metal elements such as Na and K migrate easily in a dielectric film and can cause deterioration of MOS-ULSI interface characteristics. Radioactive elements such as U and Th emit alpha rays that are often responsible for soft errors of the device. Heavy metal elements such as Fe and Ni too can cause troubles of interface connections.
In general, industrial production of ruthenium is typified by the following process:
Crude ruthenium is oxidized and dissolved by the addition of caustic potash (KOH) and potassium nitrate so that ruthenium is converted to soluble potassium ruthenate. This salt is extracted with water, and chlorine gas is injected, followed by heating to form ruthenium tetroxide. It is then collected in dilute hydrochloric acid that contains methyl alcohol. The resulting solution is evaporated to dryness and then roasted in an oxygen atmosphere to ruthenium dioxide. Further, ignition of the ruthenium dioxide in hydrogen yields metallic ruthenium.
However, the ruthenium powder made this way and available on the market is unsatisfactory as an electrode material for ferroelectric capacitors. The reason was that it has too much impurity contents of alkali metals such as Na and K, heavy metals such as Fe and Ni, and radioactive elements such as U and Th.
Attempts have, therefore, been made to refine ruthenium to higher purity. For example, Japanese Patent Application Kokai No. 8-199350 discloses a process for producing a high-purity ruthenium sputtering target. The process comprises dissolving commercially available ruthenium powder in alkali, leaching the solution in water, adding excess NaOH, saturating the mixture with chlorine gas, heating until ruthenium is separated upon evaporation as ruthenium tetroxide, allowing hydrochloric acid and a methanol solution to absorb the tetroxide, refining by repeating the evaporation cycle three times, drying the solution by reflux to solid matter so as to obtain a ruthenium hydroxide precipitate in the form of a gel, drying the precipitate and heating the dry matter in air to obtain ruthenium dioxide powder, further heating the powder in a hydrogen stream until 99.999%-pure ruthenium powder results, hot pressing the powder to a disk, and electron beam-melting the disk to remove Na, K, Ca, and Mg and forming the objective sputtering target.
The above process still has problems; complexity of operation due to many steps involved, susceptibility to contamination during the course of the process, and rather poor yield.
A more recent problem of growing concern is the fact that the steady increase in the density of semiconductor thin film conductor patterns has induced the generation of more and more unwanted particles upon deposition of thin film by sputtering.